The present invention is directed to an improved process for the stabilization of heat deformation in polyethylene terephthalate containers obtained by the injection blow-molding technique.
It is known that containers (bottles) in PET obtained by injection blow molding are subject to deformation. In addition, they are subject to deformation of a considerable extent when they are employed under high temperature conditions such as those which occur during hot-filling and sterilization operations.
The temperature of such treatments can reach 80xc2x0-90xc2x0 C. and the material tends to soften and shows shrinkage once cooled, with a volume reduction which can reach 15-250%.
With the aim of reducing or eliminating such inconveniences, the preforms of the containers obtained by injection molding are subjected to heat stabilization treatment (heat-setting), which is carried out during the blowing phase of the preform.
The treatment temperature is higher than 130xc2x0 C. and generally is between 130xc2x0 and 180xc2x0 C. with a duration of is a few seconds. The preforms thus obtained give the desired dimensional heat set qualities to the containers.
During the treatment, however, olygomers are formed which lead to fouling of the mold, and consequently, the necessity of cleaning it and therefore frequent interruptions in the production cycle.
It has now been found that it is possible to reduce or diminish the precipitation of olygomers in the preform blow molds and therefore to be able to operate with production cycles which do not require mold cleaning operations. This can be achieved by subjecting to heat setting, preforms obtained from polyethylene terephthalate (PET) or its copolymers (COPET), in which up to 15% by moles of units deriving from terephthalic acid are substituted by units deriving from isophthalic acid and/or naphthalene dicarboxylic acid, previously subjected to upgrading or polycondensation in the solid state (SSPA) in the presence of small percentages of a dianhydride of an aromatic tetracarboxylic acid.
Pyromellitic dianhydride is the preferred dianhydride. The dianhydrides are employed in quantities from 0.01 to 0.2% by weight, preferably 0.04-0.1%.
The polycondensation treatment in the solid state is conducted at temperature generally between 180xc2x0 and 220xc2x0 C. for sufficient time to increase the intrinsic viscosity of the polymer to the desired value.
The dianhydride is added to the PET or COPET in the melt state. It is convenient to dissolve it in ethylene glycol.
It was found that, employing upgraded PET and COPET as above indicated, the heat treatment of the preforms can be carried out at temperatures notably lower than those employed in the hitherto known processes. The heating temperatures of the preform blow mold are between 80xc2x0 and 130xc2x0 C. and are sufficient on confer to the containers the necessary dimensional stability. Operating under such heating conditions, mold fouling phenomena do not occur.
The duration of the treatment is of a few seconds, generally less than 10 seconds.
The container thus obtained is cooled in air, after extraction from the mold.
The preform enters the preheated mold at temperatures between 100xc2x0 and 140xc2x0 C.
The stabilized containers obtained in this way. are suitable for applications such as hot-filling.